1. Field of the Invention
The present invention relates to a magnetic resonance imaging apparatus and a magnetic resonance imaging method that obtain an image in a subject based on a magnetic resonance signal produced in the subject.
2. Description of the Related Art
To image a coronary artery based on a magnetic resonance imaging (MRI) method, there is adopted a method which uses a three-dimensional (3D) steady-state free precession (SSFP) sequence to perform imaging in a breath-hold or natural aspiration state. In case of whole-heart MR coronary angiography (WH-MRCA) for imaging a coronary artery of an entire heart in particular, a change in position of the heart involved by breathing may affect image quality.
The rate of change in position of the heart involved by breathing is reduced when the respiration level is close to the peak thereof. Thus, a method of controlling data collection in accordance with the respiration level is used. That is, for example, the position of the diaphragm on the body axis can be detected from a signal which can be obtained by subjecting an NMR signal collected in such a region R as depicted in FIG. 1 to one-dimensional Fourier transformation. Since the position of the diaphragm on the body axis periodically fluctuates in accordance with breathing, plotting the periodically detected position of the diaphragm in time series enables obtaining such a monitor signal as depicted in FIG. 2 synchronized with a respiratory movement. Data is not collected while the peak of this monitor signal is out of an allowable range between an upper threshold USL and a lower threshold LSL as shown in FIG. 2, or data collected during such a period is not used for reconfiguring an image. Further, data collected during a period that the peak of the monitor signal is in the allowable range is utilized to reconstruct an image.
In a conventional example, the allowable range is set in accordance with a breathing state of a subject before or immediately after the start of an imaging operation, and it is not changed until the imaging operation is terminated.
The above-described technology is known from, e.g., JP-A 2000-041970 (KOKAI), JP-A 2000-157507 (KOKAI), or JP-A 2004-057226 (KOKAI).
However, when a respiration level is not fixed and is gradually reduced or gradually increased and the peak of the respiration level deviates from the allowable range as shown in, e.g., FIG. 3, an efficiency for collecting data which is effective for image reconstruction is lowered, an imaging time is prolonged, or an examination cannot be terminated in the worst case.
When the allowable range is sufficiently increased, a period that a high data collection efficiency can be maintained can be extended, but an increase in influence of deformation of the heart involved by the movement of the heart may possibly degrade image quality.